Treatment of iron and steel



Patented ea 13, 1931 MI. DENNIS, OI MONTGLAIB, NEW JERSEY TBEA'IMETT OF IRON AND STEEL Ho Drawing.

This invention relates to improvements in the treatment of iron and steel, and particularly the picklingand subsequent treatment of iron and steel, to prevent or reduce rusting thereof. It also includes improvements in the enamelingofiron and steel, and par-- ticularly improvements in the preparation of iron and steel articles for the enamehng process.

In preparing ironand steel for enameling, and for certam other purposes, 1t 1s customary to pickle the metal in an acid bath, usually either hydrochloric or sulphuric acid, remove most of the acid by washlng 1n water, 5 neutralize bydipping in a dilute alkaline bath and then dry. Dryin both of the bare metal and later drying o the metal which has been dipped in a ground coat, is usually accomplished by passing the articles through 90 heated chambers. Some plants, however,

are not equipped with adequate drylng equlpment, and even those which have such dr1ers sometimes find it necessary to resort to slow drying in the open air. Even when the pickled and neutralized metal is passed giving the metal a yellowish-brown color. If the drying is incomplete, or if the factory atmosphere is damp, fairly heavy rust spots will sometimes form on the metal before it is dipped in the ground coat of the enameling process. Even if the metal is in good condltion at the time it is dipped in the ground coat, rusting may occur before the ground coat has completely dried.

Although a small amount of rust, 1ron oxide, may be absored by the ground coat during burning and may have little effect beyond the formation of a dark depresslon 1n the ground enamel, the effect is deleterious and begins to cause series difiiculty if the amount of rust present is greater than can be absorbed readily and completely in the enamel layer. In its usual form rust appears in scattered and uneven spots, which are the cause ofdefects in the enamel after burning.

It has previously been recognized that rusting of the metal durin drying and before dipping can be reduce to some extent by using in the neutralizing bath a concentrathrough a drier some rust tends to form,

Application filed February 2, 1928. Serial No. 251,461.

tion of sodium carbonate somewhat higher than is necessary for more neutralization of the acid, or even better by using a solution of borax for neutralization; but due in part to the fact that these soluble salts are lar ely removed from the surface of the metal coat they may'not remain in suflicient amount and may have little effect in preventing rusting during drying of this coat. There is a further'objection, that the relatively high concentration .of sodium carbonate sometimes used for partial rust protection leaves uring application of the wet enamel groundon the dried metal an appreciable deposit which partly dissolves in the enamel slip during dipping and progressively'changes the consistency and working properties of the slip.

I have discovered that chromates can be used in the preparation of iron and steel for enameling and for other purposes, to effect a semi-permanent rust proofing of the iron and steel, reducing or eliminating rusting during handling for some little time after the preparatory treatment. In carrying out my invention, I find it advantageous to add sodium or potassium chromate or bichromate, for example, to the neutralizing bath in which the metal is treated subsequent to pickling in preparation for the enameling or other process. When bichromate is added to an alkaline solution it combines with the alkali to form chromate. In an alkaline solution therefore the result when a bichromate is added is the same as that when a chromate is added except that the alkalinity of the solution is slightly reduced.

Following such preparatory treatment in which the metal is subjected to treatment with a solution containing a chromate, the metal may be air dried or passed through a drier with elimination or substantial reduction in rusting, and usually has a much brighter is subsequently dipped. In conjunction with the-chromate a concentration under one percent of soda ash ma be satisfactorily used under favorable con itions.

Under favorable conditions rusting may be greatly reduced by using chromates with a soda ash neutralizing bath but cannot always be completely eliminated. I have discovered, however, that by using chromates in conjunction with a dilute boraxneutralizing bath it is possible to completely prevent rusting both during1 drying of the bare metal and drying of t e enamel ground coat, even though the concentrations of chromate and v borax are held quite low and are not suflicient to carr over intothe enamel bath in uantities su cient to appreciably change its 0 aracteristics. That is, by adding chromates to a borax neutralizing bath the rust reventing effect persists not only during rst drying of the metal but also during drying of the ground coat enamel and up to t e time that the ware is fired.

In carrying out my invention, I have, for

, example, added about one to two tenths of one percent of sodium bichromate to a borax neutralizing bath (containing approximately one percent of borax) in which the metal was to be dipped for a period approximating five minutes. The neutralizing solution may be used either hot or cold, but there is added advantge in using a hot solution. This time and concentration can be varied within quite wide limits. Shorter immersion in a stronger bath gives an effect similar to that of longer immersion in a weaker bath. However, ordinary shop practice limits the time usually available and I have found it advantageous to avoid concentration such that an unnecessarily large amount of chromate is left on the metal during and after drying, particularly if the metal is subjected to a high temperature drying operation. Under normal conditions, the use of the chromate concentration for the timeI have indicated by way of example is adequate substantially to prevent rusting during initial drying and any serious rusting during drying of the ground coat.

Under some circumstances, it may be useful to employ a separate chromate-containing bath, containing chromate or bichromate,

in the preparation of iron and steel articles for enameling in accordance'with my invention; and under certain conditions it may also be desirable to add the chromium compound to the pickling bath. The procedure which will ordinarily be employed will be to pickle the metal in acid, rinse off the acid and neutralize in a bath in which the chromate is added. In some plants it may be more advantageous to pickle, rinse and then neutralize most of the acid in a plain alkali bath before giving the articles the final alkaline chromate treatment. As a measure of economy and efficiency it may sometimes be desirable to use two neutralizing baths and dip the ware first in the bath which is most nearly exhausted and later dip it in the strong bath. Spraying of chromate solution on to the ware can also at times be employed.

While I do not wish to limit myself by any theoretical explanation, yet I believe that the effect of the chromate treatment is to cause the formation of a very thin protective oxide film on the surface of the metal during drying, this thin film serving to prevent further oxidation in the form of rust while at the same time not being thick enough or of a nature to appreciably affect the adhesion of the enamel to the metal during and after burning. There are some authorities who claim that a uniform surface oxidation of the metal is even desirable. Small effects of this'kind are difficult to measure with accuracy but I have found that the metal treated in accordance with my invention may be readily enameled and shows none of the diificulties which usually accompany the presence of rust.

Under favorable conditions in ordinary enamel shop practice, rusting may be confined largely to thin edges of the metal, but at best there is nearly always some rusting and this rusting at times, at least, is serious in extent and causes considerable damage. My invention provides a simple and economical method by which trouble due to rusting in the enameling process may be greatly reduced or eliminated.

While I have described the invention in 1 detail in connection with the enameling of iron and steel, for which it is particularly advantageous, certain features of the invention are of more general application to the treatment of iron and steel for other purcases completely prevent rusting, it provides chromate following pickling and before dryan economical means by which rustin and ing, and subi'iecting the thus treated articles over to an ename pitting of the metal may be retarde appreciable. periods of time. This reduction in rusting is highly desirable, even though pickling for com lete removal of rust and scale may later 1160885351? immediately preceding use, since the di culty of picklin is reduced and a smoother metal is obtained than would be the case with iron and steel which had been stored under ordinary conditions without such rust retarding treatment. I

As an example of the further application of the invention, where it is not to be employed in combination with subsequent enameling, may be mentioned the treatment of sheet iron or steel at the rolling mills where it is produced. In cases where pickling is not necessary, the sheets may be subjected to the action of the solution containing chromate to impart to it improved rust resisting properties. In some cases the solution of alkali chromates may not contain additional alkali; but it is more advantageous in many cases to employ a solution containin both alkali chromate and additional alkali, such .as sodium carbonate, and particularly alkaline solutions containing both borax and alkali chromate. Where the sheets at the rolling mill are first subjected to pickling with acid, the resulting sheets can then be subjected to the action of the bath containing alkali chromate. The treatment with the alkali chromate therefore, may be supplemental to a pickling treatment, or it may be employed without any preliminary pickling treatment, and the chromate bath may or may not contain additional alkali compounds, in addition to the alkali chromate, although it advantageously contains such alkali compounds, particularly borax. The amounts of borax and chromate can be varied, but solutions can advantageously be employed which contain. less than 2% of borax and less than 0.2% of chromate salt. a

I claim 1. The method of treatingiron and steel 3. In the enameling of iron and steel articles, the improvement which comprises reparing the articles for enameling by pickling in an acid pickling bath and subjecting the articles to treatment with an alkaline neutralizing bath containing borax and a soluble ng operation.

In testimony whereof I afiix my signature.

CLARK M. DENNIS. 

